TUAW posted a news story that PowerPC based Macs are most likely unsupported by Firefox 4

While it may not come as much of a surprise, it's still going to aggravate owners of aging PPC Macs that the upcoming Firefox 4 browser is probably going to drop support for the older architecture. Mozilla program manager Mike Beltzner noted that the next version of the open-source browser, currently in beta, is not working on PowerPC machines now and the update will not be automatically offered to them.

  PPC Macs likely to be unsupported by Firefox 4